Multilayer reflective polarizer

ABSTRACT

Multilayer reflective polarizers are described. More particularly, multilayer reflective polarizers having a higher block light transmission at longer wavelengths than shorter wavelengths while having a high pass light transmission are described. The described multilayer reflective polarizers may be combined with absorbing polarizers or used in display devices.

BACKGROUND

Reflective polarizers substantially reflect light of one polarizationwhile substantially transmitting light of an orthogonal polarization.Multilayer optical films are formed by coextruding tens to hundreds ofmolten polymer layers and subsequently orientating or stretching theresulting film.

SUMMARY

In one aspect, the present disclosure relates to reflective polarizerssubstantially transmitting pass light and substantially reflecting blocklight. In particular, the present disclosure relates to reflectivepolarizers where an average transmission of block light at normalincidence between 600 and 750 nm is about 1.5 times or greater anaverage transmission of block light at normal incidence between 420 and600 nm, and for a range between 400 and 680 nm, a transmission of passlight as measured at a 60° angle of incidence is not less than 90%. Insome embodiments, the reflective polarizer has an average transmissionof block light at normal incidence between 600 and 750 nm about 1.8times or greater an average transmission of block light at normalincidence between 420 and 600 nm.

In another aspect, the present disclosure relates to a reflectivepolarizer where an average transmission of block light at normalincidence between 600 and 750 nm is about 1.25 or greater an averagetransmission of block light at normal incidence between 400 and 600 nm.For a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° incidence is not less than 90%. In some embodiments theaverage transmission of block light at normal incidence between 600 and750 nm is about 1.5 times or greater an average transmission of blocklight at normal incidence between 400 and 600 nm. In some embodiments, areflective polarizer has, for a range between 400 and 600 nm, an averagetransmission of block light about 5% or less at normal incidence. Insome embodiments, a reflective polarizer has, for a range between 420and 600 nm, an average transmission of block light about 5% or less atnormal incidence.

In yet another aspect, the present disclosure relates to a reflectivepolarizer where, for a range between 600 and 750 nm, an averagetransmission of block light is about 5% or greater at normal incidenceand for a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° angle of incidence in not less than 90%. In someembodiments, for a range between 400 and 600 nm, an average transmissionof block light is about 5% or less at normal incidence. In someembodiments, for a range between 600 and 680 nm, an average transmissionof light is 4% or greater at normal incidence. In some embodiments, fora range between 680 and 730 nm, an average transmission of block lightis about 8% or greater at normal incidence.

In another aspect, the present disclosure relates to a reflectivepolarizer, where, for a range between 400 and 600 nm, an averagetransmission of block light is about 5% or less at normal incidence, fora range between 600 and 680 nm, an average transmission of block lightis about 4% or greater at normal incidence, for a range between 680 and730, an average transmission of block light is about 8% or greater atnormal incidence, and for a range between 730 and 780 nm, an averagetransmission of block light is about 10% or greater at normal incidence.For a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° angle of incidence is not less than 90%.

In yet another aspect, the present disclosure relates to a reflectivepolarizer, where, for a range between 420 and 750 nm, an averagetransmission of block light is about 4.5% or greater but no greater than12% at normal incidence. For a range between 400 and 680 nm, atransmission of pass light as measured at 60° angle of incidence is notless than 90%.

In another aspect, the present disclosure relates to a reflectivepolarizer, where, for a range between 730 and 780 nm, an averagetransmission of block light at normal incidence is about 10% or greaterbut no greater than 30%, and for a range between and 680 nm, atransmission of pass light as measured at 60° angle of incidence is notless than 90%. In some embodiments, for a range between 600 and 680 nm,an average transmission of block light at normal incidence is 4% orgreater but no greater than 15%. In some embodiments, for a rangebetween 680 and 730 nm, an average transmission of block light at normalincidence is about 8% or greater but no greater than 25%.

In yet another aspect, the present disclosure relates to reflectivepolarizers where an average transmission of block light at normalincidence between 600 and 750 nm is about 1.5 times or greater anaverage transmission of block light at normal incidence between 420 and600 nm, and for a range between 400 and 680 nm, a transmission of passlight as measured at a 60° angle of incidence is greater than or equalto a transmission of pass light as measured at normal incidence.

In some embodiments, the reflective polarizer is thinner than 26 μm. Insome embodiments, the reflective polarizer is included in an opticalstack. In some embodiments, the optical stack further includes anabsorbing polarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the layer profile of Comparative Example C1.

FIG. 2 is a graph depicting the pass and block state spectra ofComparative Example C1.

FIG. 3 is a graph depicting the layer profile of Comparative Example C2.

FIG. 4 is a graph depicting the pass and block state spectra ofComparative Example C2.

FIG. 5 is a graph depicting the layer profile of Comparative Example C3.

FIG. 6 is a graph depicting the pass and block state spectra ofComparative Example C3.

FIG. 7 is a graph depicting the layer profile of Comparative Example C4.

FIG. 8 is a graph depicting the pass and block state spectra ofComparative Example C4.

FIG. 9 is a graph depicting the layer profile of Comparative Example C5.

FIG. 10 is a graph depicting the pass and block state spectra ofComparative Example C5.

FIG. 11 is a graph depicting the layer profile of Example 1.

FIG. 12 is a graph depicting the pass and block state spectra of Example1.

FIG. 13 is a graph depicting the pass and block state spectra of Example2.

FIG. 14 is a graph depicting the layer profile of Example 3.

FIG. 15 is a graph depicting the pass and block state spectra of Example3.

FIG. 16 is a graph depicting the layer profile of Example 4.

FIG. 17 is a graph depicting the pass and block state spectra of Example4.

DETAILED DESCRIPTION

Multilayer optical films, i.e., films that provide desirabletransmission and/or reflection properties at least partially by anarrangement of microlayers of differing refractive index, are known. Ithas been known to make such multilayer optical films by depositing asequence of inorganic materials in optically thin layers (“microlayers”)on a substrate in a vacuum chamber. Inorganic multilayer optical filmsare described, for example, in textbooks by H. A. Macleod, Thin-FilmOptical Filters, 2nd Ed., Macmillan Publishing Co. (1986) and by A.Thelan, Design of Optical Interference Filters, McGraw-Hill, Inc.(1989).

Multilayer optical films have also been demonstrated by coextrusion ofalternating polymer layers. See, e.g., U.S. Pat. No. 3,610,729 (Rogers),U.S. Pat. No. 4,446,305 (Rogers et al.), U.S. Pat. No. 4,540,623 (Im etal.), U.S. Pat. No. 5,448,404 (Schrenk et al.), and U.S. Pat. No.5,882,774 (Jonza et al.). In these polymeric multilayer optical films,polymer materials are used predominantly or exclusively in the makeup ofthe individual layers. Such films are compatible with high volumemanufacturing processes and can be made in large sheets and roll goods.

A multilayer optical film includes individual microlayers havingdifferent refractive index characteristics so that some light isreflected at interfaces between adjacent microlayers. The microlayersare sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference in orderto give the multilayer optical film the desired reflective ortransmissive properties. For multilayer optical films designed toreflect light at ultraviolet, visible, or near-infrared wavelengths,each microlayer generally has an optical thickness (a physical thicknessmultiplied by refractive index) of less than about 1 μm. Thicker layersmay be included, such as skin layers at the outer surfaces of themultilayer optical film, or protective boundary layers (PBLs) disposedwithin the multilayer optical films, that separate coherent groupings(referred to herein as “packets”) of microlayers.

For polarizing applications, e.g., for reflective polarizers, at leastsome of the optical layers are formed using birefringent polymers, inwhich the polymer's index of refraction has differing values alongorthogonal Cartesian axes of the polymer. Generally, birefringentpolymer microlayers have their orthogonal Cartesian axes defined by thenormal to the layer plane (z-axis), with the x-axis and y-axis lyingwithin the layer plane. Birefringent polymers can also be used innon-polarizing applications.

In some cases, the microlayers have thicknesses and refractive indexvalues corresponding to a ¼-wave stack, i.e., arranged in optical repeatunits or unit cells each having two adjacent microlayers of equaloptical thickness (f-ratio=50%), such optical repeat unit beingeffective to reflect by constructive interference light whose wavelengthλ is twice the overall optical thickness of the optical repeat unit.Other layer arrangements, such as multilayer optical films having2-microlayer optical repeat units whose f-ratio is different from 50%,or films whose optical repeat units include more than two microlayers,are also known. These optical repeat unit designs can be configured toreduce or to increase certain higher-order reflections. See, e.g., U.S.Pat. No. 5,360,659 (Arends et al.) and U.S. Pat. No. 5,103,337 (Schrenket al.). Thickness gradients along a thickness axis of the film (e.g.,the z-axis) can be used to provide a widened reflection band, such as areflection band that extends over the entire human visible region andinto the near infrared so that as the band shifts to shorter wavelengthsat oblique incidence angles the microlayer stack continues to reflectover the entire visible spectrum. Thickness gradients tailored tosharpen band edges, i.e., the wavelength transition between highreflection and high transmission, are discussed in U.S. Pat. No.6,157,490 (Wheatley et al.).

Further details of multilayer optical films and related designs andconstructions are discussed in U.S. Pat. No. 5,882,774 (Jonza et al.)and U.S. Pat. No. 6,531,230 (Weber et al.), PCT Publications WO 95/17303(Ouderkirk et al.) and WO 99/39224 (Ouderkirk et al.), and thepublication entitled “Giant Birefringent Optics in Multilayer PolymerMirrors”, Science, Vol. 287, March 2000 (Weber et al.). The multilayeroptical films and related articles can include additional layers andcoatings selected for their optical, mechanical, and/or chemicalproperties. For example, a UV absorbing layer can be added at theincident side of the film to protect components from degradation causedby UV light. The multilayer optical films can be attached tomechanically reinforcing layers using a UV-curable acrylate adhesive orother suitable material. Such reinforcing layers may comprise polymerssuch as PET or polycarbonate, and may also include structured surfacesthat provide optical function such as light diffusion or collimation,e.g. by the use of beads or prisms. Additional layers and coatings canalso include scratch resistant layers, tear resistant layers, andstiffening agents. See, e.g., U.S. Pat. No. 6,368,699 (Gilbert et al.).Methods and devices for making multilayer optical films are discussed inU.S. Pat. No. 6,783,349 (Neavin et al.).

The reflective and transmissive properties of multilayer optical filmare a function of the refractive indices of the respective microlayersand the thicknesses and thickness distribution of the microlayers. Eachmicrolayer can be characterized at least in localized positions in thefilm by in-plane refractive indices n_(x), n_(y), and a refractive indexn_(z) associated with a thickness axis of the film. These indicesrepresent the refractive index of the subject material for lightpolarized along mutually orthogonal x-, y-, and z-axes, respectively.For ease of explanation in the present patent application, unlessotherwise specified, the x-, y-, and z-axes are assumed to be localCartesian coordinates applicable to any point of interest on amultilayer optical film, in which the microlayers extend parallel to thex-y plane, and wherein the x-axis is oriented within the plane of thefilm to maximize the magnitude of Δn_(x). Hence, the magnitude of Δn_(y)can be equal to or less than—but not greater than—the magnitude ofΔn_(x). Furthermore, the selection of which material layer to begin within calculating the differences Δn_(x), Δn_(y), Δn_(z) is dictated byrequiring that Δn_(x) be non-negative. In other words, the refractiveindex differences between two layers forming an interface areΔn_(j)=n_(ij)−n_(2j), where j=x, y, or z and where the layerdesignations 1,2 are chosen so that n_(1x)≧n_(2x), i.e., Δn_(x)≧0.

In practice, the refractive indices are controlled by judiciousmaterials selection and processing conditions. A multilayer film is madeby co-extrusion of a large number, e.g. tens or hundreds of layers oftwo alternating polymers A, B, typically followed by passing themultilayer extrudate through one or more multiplication die, and thenstretching or otherwise orienting the extrudate to form a final film.The resulting film is typically composed of many hundreds of individualmicrolayers whose thicknesses and refractive indices are tailored toprovide one or more reflection bands in desired region(s) of thespectrum, such as in the visible or near infrared. To achieve highreflectivities with a reasonable number of layers, adjacent microlayerstypically exhibit a difference in refractive index (Δn_(x)) for lightpolarized along the x-axis of at least 0.05. In some embodiments,materials are selected such that the difference in refractive index forlight polarized along the x-axis is as high as possible afterorientation. If the high reflectivity is desired for two orthogonalpolarizations, then the adjacent microlayers also can be made to exhibita difference in refractive index (Δn_(y)) for light polarized along they-axis of at least 0.05.

The '774 (Jonza et al.) patent referenced above describes, among otherthings, how the refractive index difference (Δn_(z)) between adjacentmicrolayers for light polarized along the z-axis can be tailored toachieve desirable reflectivity properties for the p-polarizationcomponent of obliquely incident light. To maintain high reflectivity ofp-polarized light at oblique angles of incidence, the z-index mismatchΔn_(z) between microlayers can be controlled to be substantially lessthan the maximum in-plane refractive index difference Δn_(x), such thatΔn_(z)≦0.5*Δn_(x), or Δn_(z)≦0.25*Δn_(x). A zero or near zero magnitudez-index mismatch yields interfaces between microlayers whosereflectivity for p-polarized light is constant or near constant as afunction of incidence angle. Furthermore, the z-index mismatch Δnz canbe controlled to have the opposite polarity compared to the in-planeindex difference Δn_(x), i.e. Δn_(z)<0. This condition yields interfaceswhose reflectivity for p-polarized light increases with increasingangles of incidence, as is the case for s-polarized light.

The '774 (Jonza et al.) patent also discusses certain designconsiderations relating to multilayer optical films configured aspolarizers, referred to as multilayer reflecting or reflectivepolarizers. In many applications, the ideal reflecting polarizer hashigh reflectance along one axis (the “extinction” or “block” axis) andzero reflectance along the other axis (the “transmission” or “pass”axis). For the purposes of this application, light whose polarizationstate is substantially aligned with the pass axis or transmission axisis referred to as pass light and light whose polarization state issubstantially aligned with the block axis or extinction axis is referredto as block light. Unless otherwise indicated, pass light at 60°incidence is measured in p-polarized pass light. If some reflectivityoccurs along the transmission axis, the efficiency of the polarizer atoff-normal angles may be reduced, and if the reflectivity is differentfor various wavelengths, color may be introduced into the transmittedlight. Furthermore, exact matching of the two y indices and the two zindices may not be possible in some multilayer systems, and if thez-axis indices are not matched, introduction of a slight mismatch may bedesired for in-plane indices n1y and n2y. In particular, by arrangingthe y-index mismatch to have the same sign as the z-index mismatch, aBrewster effect is produced at the interfaces of the microlayers, tominimize off-axis reflectivity, and therefore off-axis color, along thetransmission axis of the multilayer reflecting polarizer.

Another design consideration discussed in '774 (Jonza et al.) relates tosurface reflections at the air interfaces of the multilayer reflectingpolarizer. Unless the polarizer is laminated on both sides to anexisting glass component or to another existing film with clear opticaladhesive, such surface reflections will reduce the transmission of lightof the desired polarization in the optical system. Thus, in some casesit may be useful to add an antireflection (AR) coating to the reflectingpolarizer.

Reflective polarizers are often used in visual display systems such asliquid crystal displays. These systems—now found in a wide variety ofelectronic devices such as mobile phones, computers including tablets,notebooks, and subnotebooks, and some flat panel TVs—use a liquidcrystal (LC) panel illuminated from behind with an extended areabacklight. The reflective polarizer is placed over or otherwiseincorporated into the backlight to transmit light of a polarizationstate useable by the LC panel from the backlight to the LC panel. Lightof an orthogonal polarization state, which is not useable by the LCpanel, is reflected back into the backlight, where it can eventually bereflected back towards the LC panel and at least partially converted tothe useable polarization state, thus “recycling” light that wouldnormally be lost, and increasing the resulting brightness and overallefficiency of the display.

One measure of performance of the reflective polarizer in the context ofa display system is referred to as “gain”. The gain of a reflectivepolarizer or other optical film is a measure of how much brighter thedisplay appears to the viewer with the optical film compared to thedisplay without the optical film. More specifically, the gain of anoptical film is the ratio of the luminance of the display system (or ofa portion thereof, such as the backlight) with the optical film to theluminance of the display system without the optical film. Sinceluminance is in general a function of viewing orientation, gain is alsoa function of viewing orientation. If gain is referred to without anyindication of orientation, on-axis performance is ordinarily presumed.High gains are normally associated with reflective polarizers that havevery high reflectivity for the block axis and very high transmissivity(very low reflectivity) for the pass axis, for both normally andobliquely incident light. This is because a very high block axisreflectivity maximizes the chance that a light ray of the non-useablepolarization will be reflected back into the backlight so that it can beconverted to the useable polarization; and a very low pass axisreflectivity maximizes the chance that a light ray of the useablepolarization will pass out of the backlight towards the LC panel, withminimal loss.

Another performance measure of the reflective polarizer in the contextof a full RGB color display system is the amount of color the componentintroduces into the system, both on-axis and off-axis, as a result ofspectral non-uniformities in reflectance or transmission.

Contrast ratio—that is, the ratio of transmission for light whosepolarization axis is aligned with the pass axis of the reflectivepolarizer to transmission for light whose polarization axis is alignedwith the block axis of the reflective polarizer—is another importantmetric for quantifying the performance of a reflective polarizer. Thecontrast ratio may be measured for the reflective polarizer alone or forthe reflective polarizer incorporated into a backlight, for example, incombination with a liquid crystal display panel and an absorbingpolarizer. Contrast ratio therefore may generally be improved by higheroverall pass light transmission or lower overall block lighttransmission.

In some applications, it is desirable to create a thinner reflectivepolarizer. Note that “thinner” as used here may also refer to theability to add additional optically active (e.g., to improve opticalperformance) or inactive layers (e.g., to improve physicalcharacteristics) yet preserve the same or similar thickness. Because theoptical function of the microlayers in the reflective polarizer islinked to the specific optical thickness of each microlayer, it is oftennot possible to achieve the same optical properties simply by makingeach microlayer thinner. Another option for reducing thickness isthrough judicious layer profile control, that is, in some cases,selectively omitting certain microlayers, resulting in a thinner overallreflective polarizer. Previously, it was believed that this approachresulted in a tradeoff between overall polarizer thickness and opticalperformance; for example, contrast ratio would suffer because thereduced thickness reflective polarizer would not as effectively reflectlight across the entire band of a wavelength range of interest. In otherwords, the transmission of block light (light whose polarization axis isaligned with the block axis of the reflective polarizer) increases assome of the thickest optical layers are removed (corresponding to thelongest reflected wavelengths), and it would seem that such an increasein transmitted block light would correspond to a reduced contrast ratiowhen combined, for example, with an absorbing polarizer. Surprisingly,however, for certain multilayer reflective polarizer layer profiles,despite increased block state transmission for certain wavelengths, theoverall contrast ratio when combined with an absorbing polarizer wassimilar to or better than a thicker multilayer reflective polarizer withless block state transmission for those same wavelengths. Reflectivepolarizers described herein may be thinner than 50 μm, thinner than 30μm, thinner than 20 μm, or thinner than 17 μm.

The pass light and block light transmission spectra may be a useful wayof characterizing reflective polarizers of the present disclosure. Insome embodiments, for example, pass light transmission may be not lessthan 90% in a range from 400-600 nm or 420-600 nm. Pass lighttransmission may be measured at 60° incidence. In some embodiments, passlight transmission at 60° incidence may be equal to or greater than passlight transmission at normal incidence in a range from 400-680 nm. Blocklight spectra may have a ‘sloped’ type spectra and may be characterizedby comparing average transmissions of block light, measured at normalincidence, within different wavelength ranges. For example, an averagetransmission of block light at normal incidence between 600 and 750 nmmay be about 1.5 times greater or 1.8 times greater than an averagetransmission of block light at normal incidence between 420 and 600 nm.In some embodiments, an average transmission of block light at normalincidence between 600 and 750 nm may be about 1.25 times greater or 1.5times greater than an average transmission of block light at normalincidence between 400 and 600 nm. In some embodiments, it may be helpfulto place one or more bounds on the average transmission of block lightat normal incidence for certain wavelength ranges, for example, about4.5% or greater but no higher than 12% between 420 and 750 nm. In otherembodiments, an average transmission of block light at normal incidencemay be about 10% or greater but no greater than 30% between 730 and 780nm.

Reflective polarizers of the present disclosure may be suitable forinclusion in various display devices, in some cases in combination withone or more absorbing polarizers, reflectors, turning films, prismfilms, substrates, lightguides, liquid crystal displays, or diffusers.The present disclosure also contemplates optical stacks and backlightsincluding the described reflective polarizers.

EXAMPLES Comparative Example C1

A birefringent reflective polarizer was prepared as follows. Threemultilayer optical film packets were co-extruded as described in theExample of U.S. Pat. No. 6,088,159 (Weber et al.). Polymers generallydescribed in U.S. Pat. No. 6,352,761 (Hebrink et al.) were used for theoptical layers. The first polymer (first optical layers) waspolyethylene naphthalate (PEN) homopolymer (100 mol % naphthalenedicarboxylate with 100 mol % ethylene glycol) having a Tg of 121-123° C.The second polymer (second optical layers) was a first polyethylenenaphthalate copolymer (coPEN) having 55 mol % naphthalate and 45 mol %terephthalate as carboxylates and 95.8 mol % ethylene glycol, 4 mol %hexane diol, and 0.2 mol % trimethylol propane as glycols. The secondpolymer had a Tg of 94° C. The polymer used for the skin layers was asecond coPEN having 75 mol % naphthalate and 25 mol % terephthalate ascarboxylates and 95.8 mol % ethylene glycol, 4 mol % hexane diol, and0.2 mol % trimethylol propane as glycols. The second polymer had a Tg of101° C.

The reflective polarizer was manufactured using the feedblock methoddescribed in U.S. Patent Application Publication No. 2011/0272849entitled “Feedblock for Manufacturing Multilayer Polymeric Films”. ThePEN and first coPEN polymers were fed from separate extruders to amultilayer coextrusion feedblock, in which they were assembled into apacket of 275 alternating optical layers, plus thicker protectiveboundary layers of the coPEN material on each side, for a total of 277layers. From the feedblock, the multilayer melt was conveyed through onethree-fold layer multiplier, resulting in a construction having 829layers. The skin layers of the second coPEN were added to theconstruction, resulting in a final construction having 831 layers. Themultilayer melt was then cast through a film die onto a chill roll, inthe conventional manner for polyester films, upon which it was quenched.The cast web was then stretched in a commercial scale linear tenter attemperatures and draw profiles similar to those described in Example 2Aof US Patent Application Publication No. 2007/0047080 (Stover et al.).During the production of the multilayered film a layer profile for eachpacket was targeted to best balance optical performance andmanufacturing efficiency. This layer profile is shown in FIG. 1,including first packet 110, second packet 120, and third packet 130. Theresulting pass and block state transmission spectra are shown in FIG. 2,including block light at normal incidence as curve 210, pass light at60° as curve 220, and pass light at normal incidence as curve 230. Thefilm had a thickness as measured by a capacitance gauge of approximately92 μm.

Comparative Example C2

A birefringent reflective polarizer was prepared as follows. A singlemultilayer optical packet was co-extruded as described in US PatentApplication Publication No. 2011/0102891, entitled “Low Layer CountReflective Polarizer with Optimized Gain”. Polymers generally describedin U.S. Pat. No. 6,352,761 (Hebrink et al.) were used for the opticallayers. The first polymer (first optical layers) was polyethylenenaphthalate (PEN) homopolymer (100 mol % naphthalene dicarboxylate with100 mol % ethylene glycol) having a Tg of 121-123° C. The second polymer(second optical layers) was a blend of a first polyethylene naphthalatecopolymer (coPEN) having 90 mol % naphthalate and 10 mol % copolyestersincluding Polyethylene Terephthalate Glycol (PETG) at a ratio ofapproximately 45 mol % 90/10 PEN to 55 mol % PETG. The second polymerhad a Tg of approximately 97-100° C. The polymer used for the skinlayers was the same as that used for the second polymer layers.

The reflective polarizer was manufactured using the feedblock methoddescribed in U.S. Patent Application Publication No. 2011/0272849entitled “Feedblock for Manufacturing Multilayer Polymeric Films”. Thematerials were fed from separate extruders to a multilayer coextrusionfeedblock, in which they were assembled into a packet of 305 alternatingoptical layers, plus thicker protective boundary layers formed from thesecond optical layer material on each side for a total of 307 layers.Skin layers formed from the second optical layer material were added tothe construction, resulting in a final construction having 307 layers.The multilayer melt was then cast through a film die onto a chill roll,in the conventional manner for polyester films, upon which it wasquenched. The cast web was then stretched in a commercial scale lineartenter at temperatures and draw profile similar to that described inExample 2A of US Patent Application Publication No. 2007/0047080 (Stoveret al.). During the production of the multilayered film a layer profilefor each packet was targeted to best balance optical performance andmanufacturing efficiency. This layer profile is shown in FIG. 3. Theresulting pass and block state transmission spectra are shown in FIG. 4,including block light at normal incidence as curve 410 and pass light at60° as curve 420. The film had a resulting thickness as measured by acapacitance gauge of approximately 35 μm.

Comparative Example C3

A birefringent reflective polarizer was prepared in a manner similar tothat of Comparative Example 2 as follows. A single multilayer opticalpacket was co-extruded. The packet included of 275 alternating layers of90/10 coPEN, a polymer composed of 90% polyethylene naphthalate (PEN)and 10% polyethylene terephthalate (PET), and a low index isotropiclayer, which was made with a blend of polycarbonate and copolyesters(PC:coPET). The low index layer had a refractive index of about 1.57 andremained substantially isotropic upon uniaxial orientation. The PC:coPETmolar ratio was approximately 42.5 mol % polycarbonate and 57.5 mol %coPET and the material had a Tg of 105° C. This isotropic material waschosen such that after stretching its refractive indices in the twonon-stretch directions remained substantially matched with those of thebirefringent material in the non-stretching direction, while in thestretching direction there was a substantial mis-match in refractiveindices between birefringent and non-birefringent layers.

The 90/10 PEN and PC:coPET polymers were fed from separate extruders toa multilayer coextrusion feedblock, in which they were assembled into apacket of 275 alternating optical layers, plus thicker protectiveboundary layers of the PC:coPET polymer on each side, for a total of 277layers. After the feedblock, skin layers were added where the polymerused for the skin layers was a second PC:coPET having a molar ratio of50 mol % PC and 50 mol % coPET and having a Tg of 110° C. The multilayermelt was then cast through a film die onto a chill roll, in theconventional manner for polyester films, upon which it was quenched. Thecast web was then stretched in a parabolic tenter as described in U.S.Pat. No. 7,104,776 (Merrill et al.) at temperatures and draw ratios(about 6.0) similar to that described in Example 2A of US PatentApplication Publication No. 2007/0047080 (Stover et al.).

During the production of the multilayered film a linear layer profilefor the single packet was targeted to best balance optical performanceand manufacturing efficiency. This layer profile is shown in FIG. 5. Thetargeted slope was approximately 0.24 nm/layer. The resulting pass andblock state transmission spectra are shown in FIG. 6, including blocklight at normal incidence as curve 610, pass light at 60° as curve 620,and pass light at normal incidence as curve 630. The film had aresulting thickness as measured by a capacitance gauge of approximately26.5 μm.

Comparative Example C4

A birefringent reflective polarizer was prepared in a manner similar toComparative Example C3 as follows. A single multilayer optical packetwas co-extruded. The packet included 183 alternating layers of 90/10coPEN, a polymer composed of 90% polyethylene naphthalate (PEN) and 10%polyethylene terephthalate (PET), and a low index isotropic layer. Thelow index layer was made with a blend of polycarbonate and copolyesters(PC:coPET) such that the refractive index was about 1.57 and thematerial remained substantially isotropic upon uniaxial orientation. ThePC:coPET molar ratio was approximately 42.5 mol % polycarbonate and 57.5mol % coPET and had a Tg of 105° C. This isotropic material was chosensuch that after stretching its refractive indices in the two non-stretchdirections remained substantially matched with those of the birefringentmaterial in the non-stretching direction while in the stretchingdirection there was a substantial mis-match in refractive indicesbetween birefringent and non-birefringent layers. The draw ratio used inthe parabolic tenter was about 6.5.

The 90/10 PEN and PC:coPET polymers were fed from separate extruders toa multilayer coextrusion feedblock, in which they were assembled into apacket of 183 alternating optical layers, plus thicker protectiveboundary layers of the PC:coPET material on each side, for a total of185 layers. During the production of the multilayered film a linearlayer profile for the single packet was targeted to best balance opticalperformance and manufacturing efficiency. This layer profile is shown inFIG. 7. The targeted slope was approximately 0.34 nm/layer. Theresulting pass and block state transmission spectra are shown below inFIG. 8, including block light at normal incidence as curve 810, passlight at 60° as curve 820, and pass light at normal incidence as curve830. The film had a resulting physical thickness as measured by acapacitance gauge of approximately 16.5 μm.

Comparative Example C5

A birefringent reflective polarizer was prepared as in ComparativeExample C4, except that the layer thickness profile was chosen as shownin FIG. 9. The profile was approximately linear with a targeted slope ofapproximately 0.40 nm/layer. The resulting pass and block statetransmission spectra are shown in FIG. 10, including block light atnormal incidence as curve 1010, pass light at 60° as curve 1020, andpass light at normal incidence as curve 1030. The film had a resultingphysical thickness as measured by a capacitance gauge of approximately16.3 μm. The thickness reduction was due to the broadening of the blockstate spectra thus placing more layers on average at lower wavelengths.

Example 1

A reflective polarizer including 183 alternating optical layers wasproduced in a manner similar to that described in Comparative Example C4except that the layer profile was modified as shown in FIG. 11. As shownin FIG. 12, which includes block light at normal incidence as curve1210, pass light at 60° as curve 1220, and pass light at normalincidence as curve 1230, this resulted in a sloped block state spectraand in pass state spectra that remained substantially flat for both 0degree and 60 degree incident angles. The targeted slope for layers 1through 150 was approximately 0.33 nm/layer and for layers 151 through183, the targeted slope was approximately 0.80 nm/layer. Note that theslope targeted for layers 1 through 150 was approximately the same asthat for Comparative Example C4, while the slope for layers 151 through183 was more than twice this amount. The film had a resulting thicknessas measured by a capacitance gauge of approximately 16.3 μm.

Example 2

A reflective polarizer having 183 optical layers was produced in asimilar manner to that described in Example 1 except that the draw ratiowas lowered from about 6.5 to about 6.0-6.2 in order to create lessblock state transmission while maintaining a similar sloped block statespectra. The pass and block state spectra are shown in FIG. 13,including block light at normal incidence as curve 1310, pass light at60° as curve 1320, and pass light at normal incidence as curve 1330. Thefilm had a resulting physical thickness as measured by a capacitancegauge of approximately 16.3 μm.

Example 3

A reflective polarizer having 183 optical layers was produced in asimilar manner to that described in Example 1 except that the layerprofile was modified as shown in FIG. 14. The spectra are shown in FIG.15, including block light at normal incidence as curve 1510 and passlight at 60° as curve 1520, where it can be seen that the pass stateremained substantially flat for 60 degree spectra while the block statespectra at normal incidence was sloped. The slope targeted for layers 1thru 150 was about 0.33 nm/layer while for layers 151-183 the targetedslope was about 0.90 nm/layer. The film had a resulting thickness asmeasured by a capacitance gauge of approximately 16.3 μm.

Example 4

A reflective polarizer having 183 optical layers was produced in asimilar manner to that described in Example 1 except that the layerprofile was modified as shown in FIG. 16. The spectra are shown in FIG.17 including block light at normal incidence as curve 1710, pass lightat 60° as curve 1720, and pass light at normal incidence as curve 1730,where it can be seen that the pass state spectra remained substantiallyflat for both 0 degree and 60 degrees while the block state spectra wassloped. The slope targeted for layers 1 through 150 was about 0.30nm/layer, while the targeted slope for layers 151-183 was about 0.90nm/layer. The film had a resulting thickness as measured by acapacitance gauge of approximately 16.3 μm.

Table 1 shows the average cross-web percent transmission values for thevarious Examples for the block state at 0 degrees. Table 2 shows theaverage pass state transmission values at 0 degrees and the differencein transmission values between two selected wavelength ranges. Table 3shows the average pass state transmission values at 60 degrees and thedifference in transmission values between two selected wavelengthranges. Comparative Examples C1 and C2, which possess a ‘sloping’ typespectra, have a more dramatic change in pass state transmission as afunction of angle than do Comparative Examples C3 through C5 andExamples 1-4.

The data show that by adjusting the layer profile to target a slopedspectra improved block state transmission can be achieved whilemaintaining a high and ‘flat’ pass state transmission. When combinedwith an absorbing polarizer, this can allow the absorbing polarizer toreduce contrast ratio and yet improve brightness. Alternatively,contrast ratio can be increased while maintaining system brightness indisplay devices as implied by the data in Tables 2 and 3.

TABLE 1 Block State Percent Transmission at 0 degree (%) WavelengthRange (nm) 400-600 420-600 420-750 600-680 600-750 600-780 650-780680-730 730-780 Comp. Example C1 4.73 4.41 9.13 12.20 14.78 15.51 18.3217.93 18.41 Comp. Example C2 5.42 5.63 8.17 6.52 11.20 14.35 17.80 14.9426.27 Comp. Example C3 2.34 2.25 2.02 1.50 1.74 2.33 2.63 1.55 4.41Comp. Example C4 3.93 3.78 4.07 4.67 4.43 4.27 4.08 4.27 3.65 Comp.Example C5 4.25 4.26 4.23 4.65 4.19 4.44 4.30 4.19 4.33 Example 1 2.962.96 5.66 4.61 8.38 9.61 11.83 14.33 15.61 Example 2 3.62 3.19 6.22 5.499.84 10.82 13.25 14.24 15.97 Example 3 3.47 3.26 4.99 5.02 7.05 8.7010.40 8.39 14.83 Example 4 3.84 3.30 7.03 5.81 11.48 13.45 17.03 18.4020.83

TABLE 2 Difference in Pass State Pass State Transmission at 0 degree (%)Transmission Ranges (%) Wavelength Range (nm) [400-500] [500-600][600-700] [700-800] Δ [700-800] −[400-500] Δ [600-700] −[400-500] Comp.Ex. 88.40 89.19 89.53 89.78 1.38 1.13 C1 Comp. Ex. 88.03 88.61 90.8890.80 2.77 2.85 C2 Comp. Ex. 88.41 89.24 90.13 91.04 2.62 1.72 C3 Comp.Ex. 89.71 90.20 90.72 91.06 1.35 1.01 C4 Comp. Ex. 89.79 90.31 90.9691.06 1.27 1.18 C5 Example 1 89.52 90.35 90.66 90.99 1.47 1.13 Example 289.33 90.17 90.63 90.68 1.35 1.30 Example 3 89.92 90.44 90.84 90.86 0.930.92 Example 4 89.06 90.19 90.22 91.06 2.00 1.15

TABLE 3 Difference in Pass State Pass State Transmission at 60 degree(%) Transmission (%) Wavelength Range (nm) [400-500] [500-600] [600-700][700-800] Δ [700-800] −[400-500] Δ [600-700] −[400-500] Comp. Ex. 83.0589.20 90.41 93.01 9.96 7.36 C1 Comp. Ex. 83.18 86.27 94.68 96.95 13.7611.50 C2 Comp. Ex. 93.31 94.84 97.74 98.97 5.66 4.43 C3 Comp. Ex. 95.0196.14 97.51 98.58 3.57 2.50 C4 Comp. Ex. 94.56 95.56 97.22 98.20 3.642.66 C5 Example 1 93.80 95.84 96.86 98.34 4.55 3.07 Example 2 93.3595.51 96.74 98.11 4.75 3.39 Example 3 94.20 95.74 96.79 97.17 2.97 2.59Example 4 93.78 96.00 96.75 98.33 4.55 2.97

A commercially available tablet computer having an LCD panel wasobtained. The film behind the LCD panel in the tablet contained anabsorbing polarizer with a reflective polarizer attached with anadhesive. The reflective polarizer in the tablet was very similar to thereflective polarizer of Comparative Example 3. The reflective polarizerattached to the absorbing polarizer was removed and the variousComparative Example films and Example films were attached with anoptically clear adhesive. The display was then re-assembled with thesame back-light assembly that was received with the device. Theluminance of the display was measured as a function of polar angle usinga EZ contrast XL 88W conoscope (Model XL88W-R-111124, available fromEldim-Optics, Herouville, Saint-Clair France). The luminance data isreported in Table 4 and the in-display contrast data is shown in Table5. For both Tables and corresponding data, the % difference relative tothe ‘As-Received’ display, which is equivalent to Comparative Example 3,was calculated.

TABLE 4 Avg % Axial Avg % Diff Max. Diff in Integrated Luminance inAxial Luminance Max. Intensity Avg % Diff (nits) Luminance (nits)Luminance (nits) in Int. Inten. Comparative 392.9 97.7% 396.7 98.3%442.0 98.0% Example C1 Comparative 391.5 97.3% 392.5 97.3% 439.3 97.4%Example C2 Comparative 402.3 100.0% 403.5 100.0% 450.8 100.0% Example C3Comparative 403.6 100.3% 402.1 99.6% 450.4 99.9% Example C4 Comparative400.2 99.5% 402.1 99.6% 450.4 99.9% Example C5 Example 1 402.4 100.0%406.3 100.7% 452.0 100.3% Example 2 400.6 99.6% 403.8 100.1% 450.1 99.8%Example 3 405.2 100.7% 406.0 100.6% 451.4 100.1% Example 4 399.8 99.4%402.4 99.7% 449.8 99.8%

TABLE 5 Avg % Axial Avg % Diff Max. Diff in Integrated Avg % DiffLuminance in Axial Luminance Max. Intensity in Int. Inten. CR Lum. CR CRLum. CR CR CR Comparative 884.6 100.2% 895.6 100.7% 1162 101.6% ExampleC1 Comparative 892.5 101.1% 901.5 101.4% 1165 101.9% Example C2Comparative 883 100.0% 889.1 100.0% 1144 100.0% Example C3 Comparative880.5 99.7% 902.5 101.5% 1141 99.8% Example C4 Comparative 880.2 99.7%888.8 100.0% 1140 99.6% Example C5 Example 1 896 101.5% 918.5 103.3%1162 101.6% Example 2 895.2 101.4% 903.6 101.6% 1162 101.6% Example 3901.3 102.1% 906.4 101.9% 1162 101.5% Example 4 896.1 101.5% 903.9101.7% 1167 102.0%

The following are exemplary embodiments according to the presentdisclosure:

Item 1. A reflective polarizer substantially transmitting pass light andsubstantially reflecting block light, whereinan average transmission of block light at normal incidence between 600and 750 nm is about 1.5 times or greater an average transmission ofblock light at normal incidence between 420 and 600 nm; andfor a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° angle of incidence is not less than 90%.Item 2. The reflective polarizer of item 1, wherein an averagetransmission of block light at normal incidence between 600 and 750 nmis about 1.8 times or greater an average transmission of block light atnormal incidence between 420 and 600 nm.Item 3. A reflective polarizer substantially transmitting pass light andsubstantially reflecting block light, whereinan average transmission of block light at normal incidence between 600and 750 nm is about 1.25 times or greater an average transmission ofblock light at normal incidence between 400 and 600 nm; andfor a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° angle of incidence is not less than 90%.Item 4. The reflective polarizer of item 3, wherein the averagetransmission of block light at normal incidence between 600 and 750 nmis about 1.5 times or greater an average transmission of block light atnormal incidence between 400 and 600 nm.Item 5. The reflective polarizer of any of items 1-4, wherein for arange between 400 and 600 nm, an average transmission of block light isabout 5% or less at normal incidence.Item 6. The reflective polarizer for any of items 1-4, wherein for arange between 420 and 600 nm, an average transmission of block light isabout 5% or less at normal incidenceItem 7. A reflective polarizer substantially transmitting pass light andsubstantially reflecting block light, wherein

-   -   for a range between 600 and 750 nm, an average transmission of        block light is about 5% or greater at normal incidence; and    -   for a range between 400 and 680 nm, a transmission of pass light        as measured at 60° angle of incidence is not less than 90%.        Item 8. The reflective polarizer of item 7, wherein for a range        between 400 and 600 nm, an average transmission of block light        is about 5% or less at normal incidence.        Item 9. The reflective polarizer of item 7 or 8, wherein for a        range between 600 and 680 nm, an average transmission of block        light is 4% or greater at normal incidence.        Item 10. The reflective polarizer of any of item 7-9, wherein        for a range between 680 and 730 nm, an average transmission of        block light is about 8% or greater at normal incidence.        Item 11. A reflective polarizer substantially transmitting pass        light and substantially reflecting block light, wherein    -   for a range between 400 and 600 nm, an average transmission of        block light is about 5% or less at normal incidence;    -   for a range between 600 and 680 nm, an average transmission of        block light is 4% or greater at normal incidence;    -   for a range between 680 and 730 nm, an average transmission of        block light is about 8% or greater at normal incidence;    -   for a range between 730 and 780 nm, an average transmission of        block light is about 10% or greater at normal incidence; and    -   for a range between 400 and 680 nm, a transmission of pass light        as measured at 60° angle of incidence is not less than 90%.        Item 12. A reflective polarizer substantially transmitting pass        light and substantially reflecting block light, wherein    -   an average transmission of block light at normal incidence        between 420 and 750 nm is about 4.5 percent or greater but no        higher than 12%; and    -   for a range between 400 and 680 nm, a transmission of pass light        as measured at 60° angle of incidence is not less than 90%.        Item 13. A reflective polarizer substantially transmitting pass        light and substantially reflecting block light, wherein    -   for a range between 730 and 780 nm, an average transmission of        block light at normal incidence is about 10% or greater but no        greater than 30%; and    -   for a range between 400 and 680 nm, a transmission of pass light        as measured at 60° angle of incidence is not less than 90%.        Item 14. The reflective polarizer of item 13, wherein for a        range between 600 and 680 nm, an average transmission of block        light at normal incidence is 4% or greater but no greater than        15%.        Item 15. The reflective polarizer of either item 13 or 14,        wherein for a range between 680 and 730 nm, an average        transmission of block light at normal incidence is about 8% or        greater but no greater than 25%.        Item 16. A reflective polarizer substantially transmitting pass        light and substantially reflecting block light, wherein    -   an average transmission of block light at normal incidence        between 600 and 750 nm is about 1.5 times or greater an average        transmission of block light at normal incidence between 420 and        600 nm; and    -   for a range between 400 and 680 nm, a transmission of pass light        as measured at 60° angle of incidence is greater than or equal        to a transmission of pass light as measured at normal incidence.        Item 17. The reflective polarizer as in any of the previous        items, wherein the reflective polarizer is thinner than 26 μm.        Item 18. An optical stack comprising the reflective polarizer of        any of the previous items.        Item 19. The optical stack of item 18, further comprising an        absorbing polarizer.        Item 20. The optical stack of either item 18 or 19, further        comprising an LCD panel.        Item 21. A backlight comprising the optical stack of any of        items 18-20.

All U.S. patents and patent applications cited in the presentapplication are incorporated herein by reference as if fully set forth.The present invention should not be considered limited to the particularexamples and embodiments described above, as such embodiments aredescribed in detail in order to facilitate explanation of variousaspects of the invention. Rather, the present invention should beunderstood to cover all aspects of the invention, including variousmodifications, equivalent processes, and alternative devices fallingwithin the scope of the invention as defined by the appended claims andtheir equivalents.

1. A reflective polarizer substantially transmitting pass light andsubstantially reflecting block light, wherein an average transmission ofblock light at normal incidence between 600 and 750 nm is about 1.5times or greater an average transmission of block light at normalincidence between 420 and 600 nm; and for a range between 400 and 680nm, a transmission of pass light as measured at 60° angle of incidenceis not less than 90%.
 2. The reflective polarizer of claim 1, wherein anaverage transmission of block light at normal incidence between 600 and750 nm is about 1.8 times or greater an average transmission of blocklight at normal incidence between 420 and 600 nm.
 3. A reflectivepolarizer substantially transmitting pass light and substantiallyreflecting block light, wherein an average transmission of block lightat normal incidence between 600 and 750 nm is about 1.25 times orgreater an average transmission of block light at normal incidencebetween 400 and 600 nm; and for a range between 400 and 680 nm, atransmission of pass light as measured at 60° angle of incidence is notless than 90%.
 4. The reflective polarizer of claim 3, wherein theaverage transmission of block light at normal incidence between 600 and750 nm is about 1.5 times or greater an average transmission of blocklight at normal incidence between 400 and 600 nm.
 5. The reflectivepolarizer of claim 3, wherein for a range between 400 and 600 nm, anaverage transmission of block light is about 5% or less at normalincidence.
 6. The reflective polarizer of claim 3, wherein for a rangebetween 420 and 600 nm, an average transmission of block light is about5% or less at normal incidence
 7. A reflective polarizer substantiallytransmitting pass light and substantially reflecting block light,wherein for a range between 600 and 750 nm, an average transmission ofblock light is about 5% or greater at normal incidence; and for a rangebetween 400 and 680 nm, a transmission of pass light as measured at 60°angle of incidence is not less than 90%.
 8. The reflective polarizer ofclaim 7, wherein for a range between 400 and 600 nm, an averagetransmission of block light is about 5% or less at normal incidence. 9.The reflective polarizer of claim 7, wherein for a range between 600 and680 nm, an average transmission of block light is 4% or greater atnormal incidence.
 10. The reflective polarizer of claim 7, wherein for arange between 680 and 730 nm, an average transmission of block light isabout 8% or greater at normal incidence.
 11. A reflective polarizersubstantially transmitting pass light and substantially reflecting blocklight, wherein for a range between 400 and 600 nm, an averagetransmission of block light is about 5% or less at normal incidence; fora range between 600 and 680 nm, an average transmission of block lightis 4% or greater at normal incidence; for a range between 680 and 730nm, an average transmission of block light is about 8% or greater atnormal incidence; for a range between 730 and 780 nm, an averagetransmission of block light is about 10% or greater at normal incidence;and for a range between 400 and 680 nm, a transmission of pass light asmeasured at 60° angle of incidence is not less than 90%.
 12. Areflective polarizer substantially transmitting pass light andsubstantially reflecting block light, wherein the reflective polarizeris thinner than 30 μm; an average transmission of block light at normalincidence between 420 and 750 nm is about 4.5 percent or greater but nohigher than 12%; and for a range between 400 and 680 nm, a transmissionof pass light as measured at 60° angle of incidence is not less than90%.
 13. A reflective polarizer substantially transmitting pass lightand substantially reflecting block light, wherein for a range between730 and 780 nm, an average transmission of block light at normalincidence is about 10% or greater but no greater than 30%; and for arange between 400 and 680 nm, a transmission of pass light as measuredat 60° angle of incidence is not less than 90%.
 14. The reflectivepolarizer of claim 13, wherein for a range between 600 and 680 nm, anaverage transmission of block light at normal incidence is 4% or greaterbut no greater than 15%.
 15. The reflective polarizer of claim 13,wherein for a range between 680 and 730 nm, an average transmission ofblock light at normal incidence is about 8% or greater but no greaterthan 25%.
 16. A reflective polarizer substantially transmitting passlight and substantially reflecting block light, wherein an averagetransmission of block light at normal incidence between 600 and 750 nmis about 1.5 times or greater an average transmission of block light atnormal incidence between 420 and 600 nm; and for a range between 400 and680 nm, a transmission of pass light as measured at 60° angle ofincidence is greater than or equal to a transmission of pass light asmeasured at normal incidence.
 17. The reflective polarizer as in claim1, wherein the reflective polarizer is thinner than 26 μm.
 18. Anoptical stack comprising the reflective polarizer of claim
 1. 19. Theoptical stack of claim 18, further comprising an absorbing polarizer.20. The optical stack of claim 19, further comprising an LCD panel. 21.A backlight comprising the optical stack of claim 18.